The present invention relates to a surface acoustic wave (SAW) device incorporating diamond, particularly a SAW device that has excellent operational performance even at frequency ranges such as gigahertz and higher-frequency bands.
As stated in the published Japanese patent application Tokukaihei 10-276061, a typical SAW device, which incorporates diamond, is known to be produced by forming a ZnO layer on a diamond layer, forming on the ZnO layer interdigital electrodes (IDTs), which excite and receive a SAW, and finally forming an SiO2 layer on the ZnO layer to enable the SiO2 layer to cover the IDTs.
The SAW device is intended to achieve not only excellent propagation, electromechanical coupling, and frequency-temperature properties but also low propagation loss by obtaining an optimum combination of-the thicknesses of the IDTs, ZnO layer, and SiO2 layer. The SAW device realizes a frequency-temperature property of xe2x88x9215 to +15 ppm/xc2x0 C. and an electromechanical coupling coefficient of 0.1 to 1.3% at a propagation velocity of 8,000 to 12,000 m/s.
However, when the conventional SAW device is planned for use at a frequency cy band as high as 10 GHz or so, even if the propagation velocity is increased to 10,000 m/s, it is necessary to reduce the combined value of the width of the digit electrodes and the distance between the neighboring digit electrodes of the IDTs to 0.5 xcexcm or so and the width of the digit electrodes to 0.25 xcexcm or so. This requirement is disadvantageous for mass production of the SAW device.
Moreover, a conventional material such as quartz has a limitation of propagation velocity at 3,150 m/s, so it cannot be used for a SAW device for the superhigh-frequency band.
Another drawback is that a SAW device reduces its electromechanical coupling coefficient at the superhigh-frequency band. For example, a SAW device made with quartz, which has an electromechanical coupling coefficient of 0.1% at the fundamental wave, reduces the coefficient to 0.025% at the fifth harmonic. A low-loss filter cannot be achieved with a small electromechanical coupling coefficient.
The present invention aims to solve the foregoing problems, and its object is to offer a SAW device that is suitable for mass production and that has excellent operational performance at the superhigh-frequency range.
A SAW device of the present invention comprises:
(a) a diamond layer;
(b) a ZnO layer, with a thickness of tz, formed on the diamond layer;
(c) IDTs, which excite and receive a SAW, formed on the ZnO layer; and
(d) an SiO2 layer, with a thickness of ts, formed on the ZnO layer so that the SiO2layer can cover the IDTs.
In order to determine the structure of the SAW device, a two-dimensional orthogonal-coordinate system is provided, in which the axis of abscissa represents kh1 and the axis of ordinate represents kh2. In the above description, kh1 and kh2 are given in the following equations:
kh1=5xc2x72xcfx80xc2x7(tz/xcex); and
kh2=5xc2x72xcfx80xc2x7(ts/xcex),
where xcex signifies the wavelength of the fundamental wave of the second mode of the SAW.
In the orthogonal-coordinate system, the range of ABCDEFGHIJKLMNOPQRSTUVWA is provided by connecting the following 23 points with 23 lengths of lines in this order:
point A given by the coordinates xe2x80x9ckh1=4.4 and kh2=7.4xe2x80x9d;
point B given by the coordinates xe2x80x9ckh1=5.0 and kh2=6.9xe2x80x9d;
point C given by the coordinates xe2x80x9ckh1=5.2 and kh2=6.2xe2x80x9d;
point D given by the coordinates xe2x80x9ckh1=5.0 and kh2=5.6xe2x80x9d;
point E given by the coordinates xe2x80x9ckh1=4.5 and kh2=5.1xe2x80x9d;
point F given by the coordinates xe2x80x9ckh1=4.0 and kh2=4.6xe2x80x9d;
point G given by the coordinates xe2x80x9ckh1=3.5 and kh2=4.4xe2x80x9d;
point H given by the coordinates xe2x80x9ckh1=3.0 and kh2=4.1xe2x80x9d;
point I given by the coordinates xe2x80x9ckh1=2.8 and kh2=4.0xe2x80x9d;
point J given by the coordinates xe2x80x9ckh1=2.6 and kh2=3.4xe2x80x9d;
point K given by the coordinates xe2x80x9ckh1=3.0 and kh2=3.0xe2x80x9d;
point L given by the coordinates xe2x80x9ckh1=3.5 and kh2=2.9xe2x80x9d;
point M given by the coordinates xe2x80x9ckh1=3.5 and kh2=2.0xe2x80x9d;
point N given by the coordinates xe2x80x9ckh1=3.0 and kh2=2.0xe2x80x9d;
point O given by the coordinates xe2x80x9ckh1=2.5 and kh2=2.0xe2x80x9d;
point P given by the coordinates xe2x80x9ckh1=2.0 and kh2=2.0xe2x80x9d;
point Q given by the coordinates xe2x80x9ckh1=1.8 and kh2=2.6xe2x80x9d;
point R given by the coordinates xe2x80x9ckh1=1.7 and kh2=4.0xe2x80x9d;
point S given by the coordinates xe2x80x9ckh1=2.0 and kh2=4.5xe2x80x9d;
point T given by the coordinates xe2x80x9ckh1=2.5 and kh2=5.2xe2x80x9d;
point U given by the coordinates xe2x80x9ckh1=3.0 and kh2=5.7xe2x80x9d;
point V given by the coordinates xe2x80x9ckh1=3.5 and kh2=6.1xe2x80x9d;
point W given by the coordinates xe2x80x9ckh1=4.0 and kh2=6.8xe2x80x9d; and
point A.
The combination of kh1 and kh2 is determined so that it can fall in the range of ABCDEFGHIJKLMNOPQRSTUVWA including the surrounding 23 lengths of lines. The SAW device uses the fifth harmonic of the second mode of the SAW.
Another SAW device of the present invention, also, comprises:
(a) a diamond layer;
(b) a ZnO layer, with a thickness of tz, formed on the diamond layer;
(c) IDTs, which excite and receive a SAW, formed on the ZnO layer; and
(d) an SiO2 layer, with a thickness of ts, formed on the ZnO layer so that the SiO2 layer can cover the IDTs.
In order to determine the structure of the SAW device, a two-dimensional orthogonal-coordinate system is provided, in which the axis of abscissa represents sent kh1 and the axis of ordinate represents kh2. In the above description, kh1 and kh2 are given in the following equations:
kh1=5xc2x72xcfx80xc2x7(tz/xcex); and
kh2=5xc2x72xcfx80xc2x7(ts/xcex),
where xcex signifies the wavelength of the fundamental wave of the second mode of the SAW.
In the orthogonal-coordinate system, the range of ABCDEFGHIJKLMNOPQRSTA is provided by connecting the following 20 points with 20 lengths of lines in this order:
point A given by the coordinates xe2x80x9ckh1=4.4 and kh2=6.9xe2x80x9d;
point B given by the coordinates xe2x80x9ckh1=5.0 and kh2=6.4xe2x80x9d;
point C given by the coordinates xe2x80x9ckh1=5.2 and kh2=6.2xe2x80x9d;
point D given by the coordinates xe2x80x9ckh1=5.0 and kh2=5.6xe2x80x9d;
point E given by the coordinates xe2x80x9ckh1=4.6 and kh2=5.2xe2x80x9d;
point F given by the coordinates xe2x80x9ckh1=4.4 and kh2=5.0xe2x80x9d;
point G given by the coordinates xe2x80x9ckh1=4.0 and kh2=4.6xe2x80x9d;
point H given by the coordinates xe2x80x9ckh1=3.5 and kh2=4.4xe2x80x9d;
point I given by the coordinates xe2x80x9ckh1=3.0 and kh2=4.1xe2x80x9d;
point J given by the coordinates xe2x80x9ckh1=2.8 and kh2=4.0xe2x80x9d;
point K given by the coordinates xe2x80x9ckh1=2.6 and kh2=3.4xe2x80x9d;
point L given by the coordinates xe2x80x9ckh1=2.8 and kh2=3.0xe2x80x9d;
point M given by the coordinates xe2x80x9ckh1=3.2 and kh2=2.4xe2x80x9d;
point N given by the coordinates xe2x80x9ckh1=2.7 and kh2=2.4xe2x80x9d;
point O given by the coordinates xe2x80x9ckh1=2.2 and kh2=3.0xe2x80x9d;
point P given by the coordinates xe2x80x9ckh1=2.2 and kh2=3.5xe2x80x9d;
point Q given by the coordinates xe2x80x9ckh1=2.5 and kh2=4.7xe2x80x9d;
point R given by the coordinates xe2x80x9ckh1=3.0 and kh2=5.2xe2x80x9d;
point S given by the coordinates xe2x80x9ckh1=3.5 and kh2=5.7xe2x80x9d;
point T given by the coordinates xe2x80x9ckh1=4.0 and kh2=6.3xe2x80x9d; and
point A.
The combination of kh1 and kh2 is determined so that it can fall in the range of ABCDEFGHIJKLMNOPQRSTA including the surrounding 20 lengths of lines. The SAW device uses the fifth harmonic of the second mode of the SAW.
A SAW device of the present invention uses the fifth harmonic of the second mode of the SAW at a propagation velocity, v, of 4,500 to 6,500 m/s. In the SAW device, the IDTs have a plurality of comb-tooth-shaped digit electrodes. When the width of the digit electrodes is expressed as dm, the distance between the neighboring digit electrodes is expressed as df, and the center frequency of the fifth harmonic of the second mode of the SAW is expressed as f0, the digit electrode""s pitch, dm+df, is expressed in the equation dm+df=(5xc2x7v)/(2xc2x7f0). (See FIG. 2.)
A SAW device of the present invention has the IDTs of which the width of the digit electrode is 0.5 xcexcm or more. In the SAW device, the center frequency of the fifth harmonic of the second mode of the SAW is 5.0 to 11.3 GHz.
Another SAW device of the present invention, also, has the IDTs of which the width of the digit electrode is 0.5 xcexcm or more. In the SAW device, however, the center frequency of the fifth harmonic of the second mode of the SAW is 9.5 to 10.5 GHz.
As mentioned above, the present invention uses the fifth harmonic of the second mode of the SAW excited by the IDTs. This enables the SAW device to obtain an excellent propagation property, electromechanical coupling coefficient, and frequency-temperature property at the superhigh-frequency range. Moreover, the present invention allows the use of wider digit electrodes in the IDTs, so that mass production of the SAW device can be easily achieved.